This invention relates generally to kaolin clays, and more specifically relates to a method for processing a kaolin clay which has relatively poor high-shear viscosity when prepared as a high solids suspension, so as to obtain therefrom a fraction having a value of high-shear viscosity in high solids suspension which is low relative to the starting clay, thereby rendering same more suitable for paper and paperboard coating applications.
Kaolin clay coating pigments having very fine particle size and high brightness characteristics, are widely utilized in the coating of merchant grade papers and various types of paperboard wherein high gloss and smoothness of coating is required. Typically, these pigments are applied as a high solids aqueous suspension, i.e., a suspension including from approximately 60-75% by weight of clay solids. The size distribution of prior art pigments used for such purposes are usually such that of the order of 90-100% by weight thereof are of less than 2 microns equivalent spherical diameter (E.S.D.). Typically, further, the brightness characteristics, as measured by the standard specification established by TAPPI procedure T-646m-54, are of the order of at least 90.
Among the further qualities of a high solids coating clay slurry, which are of paramount importance for achieving high quality coatings, are the viscosity characteristics of same. It may be noted in this connection that the term "viscosity" as used herein with respect to clay slurries, refers to such characteristics as determined by the procedures of TAPPI Method T 648 su-72, as revised in 1972. This method sets forth specific procedures for determination of both the "low shear" and "high shear" viscosity. The latter, i.e. the high shear viscosity, is considered of special importance in evaluating a high solids clay slurry for the aforementioned coating purposes.
Many crude kaolin clays are unsuitable for the production of grades of kaolin for use in the paper and paperboard coating industry. There are numerous causes of unsuitability such as the failure of off-color crudes to respond to known methods of bleaching and beneficiation. Another cause is the failure of crudes to yield products with good flow characteristics, in aqueous suspension, as demanded by the paper and paperboard coating industry for operational reasons. With reference to flow characteristics there are several causes of poor viscosity some of which are not understood. Evidence thus far gathered suggests that one cause of poor viscosity is minute aggregates of kaolin particles which are too small to be removed by normal sedimentation practices, and which are not dispersed by normal practices. One aspect of this invention relates to the fractionation of such crudes so as to yield a fraction which is substantially free from such aggregates and, consequently, has suitable flow properties for use in paper and paperboard coating.
It should be understood that the flow properties demanded by the paper coating industry have become more stringent. This is primarily due to the increase in coating solids and to the development of coating methods which involve high rates of shear during the application of the coating to the paper. This is recognized by TAPPI and indeed the mentioned TAPPI Method T 648, su 72, incorporated a high-shear method of measuring viscosity when it was revised in 1972.
It is well known that, in addition to having suitable flow properties the kaolin pigments must satisfy the papermaker relative to the gloss, brightness, and printing properties of the finished coated product. Gloss and printing properties are associated with the particle size distribution of the clay particles and in the production of coating clays the particle size of the finished product can be controlled by fractionating the clay, such as by the method disclosed in Maloney, U.S. Pat. No. 2,158,987. However, methods such as those of Maloney do not yield a fraction of good viscosity when applied to a crude of poor viscosity.
In accordance with the foregoing, it may be regarded as an object of the present invention, to provide a method which yields a product of acceptable high-shear viscosity and of the desired particle size, from a poor viscosity crude.
It is a further object of the present invention, to provide a method for beneficiating a kaolin clay to improve the high-shear viscosity of a high solids suspension subsequently formed from the beneficiated pigment.
It is a still further object of this invention to provide a method as aforesaid, which simultaneously improves the size characteristics of the clay, to markedly decrease the proportion of the clay particles which may have deleterious size or shape characteristics.